(1) Field of the Invention
The invention relates to the general field of mechanics and hydraulics.
The invention relates to damping a mechanical system and more particularly to equipment including damper systems used in aviation.
(2) Description of Related Art
The invention is applicable in any damping system, and especially in lead-lag dampers used on a main rotor of a rotorcraft, and in particular of a helicopter.
By way of example, on a “soft-in-plane” helicopter rotor, a lead-lag damper system connects each blade to the hub of said rotor or interconnects two consecutive blades. Such dampers are also fitted of necessity to rotors of the “stiff-in-plane” type, but they are subjected to less stress than in soft-in-plane rotors.
Such a damper makes it possible to avoid catastrophic coupling between the lead-lag movements of the blades and the airframe of the helicopter. Such phenomena are known as “ground resonance” and “air resonance”.
Such a damper is subjected to forced movements at the frequency of rotation of the rotor, due to the pitch movements and the flapping movements of said blade, and also to the lead-lag movements of the blade. The damper influences those movements to only a very small extent.
The damper is also subjected to movements at the specific lead-lag frequency of the blade.
These movements are not damped naturally, e.g. by aerodynamic forces. The damper therefore acts to oppose any resonance phenomenon.
Damping the lead-lag movement of the blades, with a view to guaranteeing stability for the helicopter, makes it possible to prevent the ground resonance and air resonance phenomena.
In order to generate an appropriate damping relationship, it is known to use particular dampers. Such dampers are, in general, based on a relationship that is characteristic of force as a function of dynamic movement of the damper, which relationship has three distinct damping ranges depending on the dynamic movement stress to which they are subjected. Such a relationship may be referred to as a relationship of the “triple slope” type.
Document EP 1 659 309 describes a hydraulic damper for lead-lag damping of the blades of a helicopter, that damper having hydraulic constrictions formed by calibrated orifices and by hydraulic peak-limiting valves so as to generate a damping relationship of the “triple slope” type, as presented above. The damper described in that document also has an end-of-stroke abutment made of elastomer. Unfortunately, that damper suffers from drawbacks. The presence of a mechanical abutment made of elastomer does not make it possible to avoid the occurrence of peaks of force resulting from end-of-stroke jolting of the damper. It should also be emphasized that it is difficult to adjust the peak-limiting valves. The damper needs to be removed in order to perform such adjustment, and then refitted.
Document FR 1 292 739 describes a hydraulic damper device. A base of a piston fits the bore of a throat of a cylinder, while a ring is movable axially.
Document U.S. Pat. No. 5,347,771 describes an anti-earthquake safety system, with a high-damping damper device provided with a piston. Chambers of that device are hydraulically suitable for being interconnected through the piston.
Document NL 92/02218 describes a perforated damper piston mounted to slide in a cylinder, with an arrangement integrated into the piston that, at the end-of-stroke limit, opens up a passage for discharging fluid, which passage is normally closed by sealing means.
Document U.S. Pat. No. 5,517,898 describes a pneumatic cylinder that uses damping (“cushioning”) chambers, quick fluid exhaust valves, and quick fluid supply valves.